1. Field of the Invention
The present invention relates to a droplet ejecting device for recording high-quality images quickly and reliably, a method of calibrating ejection characteristics for this droplet ejecting device, and an inkjet recording device equipped with the droplet ejecting device.
2. Description of Related Art
On-demand multi-nozzle inkjet recording heads having a plurality of integrated nozzles are well known in the art. With this type of inkjet recording head, it is important to produce ink droplets from each nozzle with a uniform ejection velocity and mass in order to record high-quality images quickly and reliably.
In an on-demand inkjet recording head having a push-type piezoelectric element system, a pressure chamber having orifices for nozzle holes includes a diaphragm serving as one wall of the pressure chamber. Bar-shaped piezoelectric elements generate longitudinal vibrations that push the diaphragm, reducing the volume in the pressure chamber and causing an ink droplet to be ejected through a nozzle hole. Conventionally, methods have been adopted to improve the precision of various components constituting the piezoelectric elements, the pressure chamber, and the like and to improve the precision for assembling such components through adhesive bonding and the like in order to reduce variations in the mass and velocity of ink droplets ejected from this recording head.
However, this method has led to such problems as an increased cost in the manufacturing of parts and an increase in assembly time. To avoid these problems, Japanese unexamined patent application publication No. 2001-277525 and others propose a method of reducing variations in the mass and velocity of ink droplets ejected from each nozzle by suitably adjusting the degree of polarization in the piezoelectric elements, that is, a polarization calibration method. This method can provide an inkjet recording head capable of improving the uniformity of ink droplet ejection, without the addition of parts or circuits, but merely adding a calibration step in the manufacturing process.
However, when calibrating the degree of polarization of the piezoelectric elements in this conventional method, it is necessary to repolarize the piezoelectric elements in an elevated temperature state. Consequently, the method requires a heating device for heating the piezoelectric elements and considerable time and effort to perform this heating, making it difficult to sufficiently reduce costs and improve productivity.
As a method for resolving these problems, it is conceivable to conduct polarization at ambient temperatures near room temperature (around 25° C.). However, the polarization of the piezoelectric elements tends to be set lower than the degree of polarization set at high temperatures. It has been determined that, when polarizing piezoelectric elements by applying a drive voltage thereto, the calibration of polarization breaks down as the drive time elapses, resulting in a wide variation of velocities among nozzles.